Openings are formed in support materials so that microelectronic structures may be formed in and their structure supported by the support material. For example, a capacitor container for a dynamic random access memory (DRAM) cell may be etched into a dielectric, such as silicon dioxide (SiO2), which can be formed as a doped silicate glass. Use of silicon dioxide dielectric can yield several disadvantages from an etching standpoint. Dry etch of silicon dioxide has a large physical component, that is, it is more like physical sputtering than like a chemical etch. Its sputtering nature creates difficulty in obtaining a straight profile since the etch does not exhibit a lateral component, leading to a tapered profile.
The profile of an opening is of particular importance. The need for accurately and precisely locating openings is in relation to other structures is exemplified where contact holes or vias are provided between devices. The profile of an opening is also important where several adjacent openings are used to form vertical capacitive structures. As the feature dimensions of devices decrease, the aspect ratio (the ratio of depth to width) of the openings tend to increase. As the aspect ratio increases, however, a phenomenon termed “twisting” can occur.
In addition to tapered etch profiles, use of silicon dioxide also may produce feature charging due to its insulative nature. Consequently, the top of a feature, such as an opening in the silicon dioxide, charges negatively relative to the bottom of the feature. Computer simulation has shown the resulting vertical potential gradient as high as several hundred volts, for example, 200 to 300 volts. Such a gradient may retard the flux of positive ions that produce the etching effect and contribute to aspect ratio dependent (ARD) etch, also known as reactive ion etch (RIE) lag. As a result, as aspect ratio increases, etching may become less effective.
Due to the physical component involved in a dry etch process of silicon dioxide (SiO2), it is also possible for a lateral potential gradient to exist. Features across a surface being etched might not be symmetrical, resulting in feature charging differences in lateral directions. Feature asymmetries may result from incoming photo irregularities, asymmetries at the edge of an array compared to the center of an array, or the stochastic nature of plasma polymer deposition. Photo irregularities become apparent on inspection after the development step during photolithography.
While a vertical electric field is responsible for aspect ratio dependent etching (ARDE), a lateral potential gradient, i.e., electric field, may orient the flux of positive ions away from true vertical, leading to so-called twisting of etched features. Twisting occurs when the etch front of the opening starts deviating from what is perpendicular to the semiconductor substrate surface, for example, openings in the shape of a corkscrew are possible. Twisting may become especially noticeable in high aspect ratio (HAR) features. When etching a HAR or other feature, openings may deflect laterally from true vertical.
The twisting phenomenon with respect to HAR openings is problematic in that twisting reduces the efficiency of a contact by increasing the distance between active device areas and by increasing the difficulty of filling a contact with a conductive material, or can weaken or distort vertical structures formed by etching. Such twisting may cause electrical opens when the opening misses a landing contact, or may cause electrical shorts when the opening twists into an adjacent feature, or may cause unwanted variations in deposition layers upon distorted vertical structures.